Conventionally, illumination systems or devices which are used to scan documents typically include an illumination source such as a tungsten lamp or fluorescent lamp and an opposing reflector. More specifically, in conventional systems, the illumination source would be located on one side of an optical center line of the scanner under a platen or constant velocity transport (CVT) system while the reflector would be positioned on the opposite side of the optical center line of the scanner under the platen or CVT system. Thus, the illumination source, in conjunction with the opposing reflector, would provide proper illumination of the document or object being scanned by the scanner. It is noted that document and object, in the present specification, are interchangeable and refer to the physical object that is being scanned.
The opposing reflector serves two distinct purposes in an illumination system as described above. The first purpose is to redirect light which would have normally missed the document back to the illuminated region of the document, thereby significantly increasing the total irradiance impinging upon the document without additional input power being applied to the lamp or light source. Moreover, the opposing reflector suppresses shadowing due to paste-ups or other surface irregularities on the document by supplying light from an angle opposite the angle corresponding to the light being directly contributed by the lamp. An example of such a conventional system is illustrated in FIG. 1.
FIG. 1 illustrates the document illumination components in a conventional scanner which can be utilized as a stand alone scanner or in a copier. Moreover, the illumination system illustrated in FIG. 1 may be utilized in either a light lens scanning system or a digital (image input terminal) scanning system (platen or CVT) so as to properly illuminate the document for reproduction, display, and/or storage in an electronic memory, magnetic medium, or optical disk. The illumination system of FIG. 1 includes a platen 3 upon which a document 1 rests. The document 1 is illuminated by a light source 7 which provides an active source of light to irradiate the illuminated region of the document. Typically, this source of light is provided by a linear fluorescent lamp (with or without aperture reflecting coatings 8 on the interior lamp surface to form an aperture), or by linear tungsten lamps. As noted above, the light source 7 is situated on one side of an optical center line 6. An imaging system (not shown) causes an image of the portion of the document immediately surrounding this optical centerline 6 to be projected onto a linear array of photosensors (full width array sensor) (not shown), a CCD sensor (not shown), or a photoreceptor (not shown).
On the opposite side of the optical center line 6, an opposing reflector 5 is situated. The opposing reflector 5 provides indirect illumination to the document being scanned by redirecting light which would normally not illuminate the document (light leaving the light source in a parallel or substantially parallel path with respect to the platen 3) back to the illuminated region of the document. Moreover, the opposing reflector 5 reflects light back to the document at an angle opposite from the angle of light being directly contributed by the light source 7. This reflection suppresses shadowing on documents with paste-up or other nonplanar surface features.
Conventionally, the opposing reflector 5 is formed from extruded aluminum or a formed piece of aluminum sheet stock. A highly reflective finish is then applied, by polishing and optionally the application of special coatings, to the aluminum to provide the reflective surface of the opposing reflector 5. It is further noted that the opposing reflector 5 may have a thin flat glass mirror attached to its surface to improve reflectivity. However, such a thin flat glass mirror limits the surface to having an essentially flat profile.
Examples of conventional document illumination systems are disclosed in U.S. Pat. No. 3,851,201; U.S. Pat. No. 3,868,182; U.S. Pat. No. 4,072,417; U.S. Pat. No. 4,218,127; and "Full Frame Array Lens Illuminator," Xerox Disclosure Journal, Volume 11, Number 1, January/February 1986. The entire contents of these U.S. patents and article are hereby incorporated by reference.
Although the conventional illumination systems provide adequate illumination for a scanner, various problems are associated with conventional illumination systems. One such problem is the mechanical tolerances required for obtaining proper reflector to lamp alignment in the scanner. More specifically, the alignment between the opposing reflector and light source must be precisely set and maintained to provide the desired illumination efficiency and profile on the document for proper scanning.
Another problem with conventional illumination systems is their reflective efficiency and left to right illumination balance. More specifically, the reflectors in conventional illumination systems utilizing polished aluminum with special coatings typically have a reflectance value of less than 70%. Thus, the opposing reflector in the conventional illumination system has a reflective efficiency of less than 70%. This efficiency has a direct effect upon the left to right illumination balance of the illumination system.
For example, it is desirable to maximize the irradiance at the document at the optical center line, and simultaneously receive nearly equal irradiance contributions from the lamp and reflector sides. If the reflectance value of the opposing reflector is reduced, the overall irradiance as well as the reflector side irradiance contribution are correspondingly reduced for a given illuminator design. This adversely impacts both the total illuminator efficiency as well as the side-to-side balance. Clearly, achieving a reflectance value of the opposing reflector surface as close to 100% as possible not only improves the overall illuminator efficiency but also improves the ability to achieve equal irradiance contributions from each side of the optical center line.
A third problem with conventional illumination systems is the repairing and maintenance of these illumination systems. More specifically, when an opposing reflector is damaged, the aluminum reflector must be either replaced or removed, repolished, and re-coated and placed back into the illumination system so as to restore the illumination system to its original performance. Thus, the repairing of the conventional illumination system can be complicated and costly.
Therefore, it is desirable to provide an illumination system for a scanner which meets tight mechanical tolerances required for maintaining proper reflector to lamp alignment, has high reflective efficiency so as to realize optimum left to right illumination balance, and a simplified design to reduce the complexity and cost of repairing of such an illumination system.